Invertases mediate the hydrolysis of sucrose into glucose and fructose which are the central molecules for carbohydrate translocation, metabolism and sensing in higher plants. Plants possess three types of invertases, which are located in the apoplast, the cytoplasm and the vacuole. Extracellular and vacuolar invertase isoenzymes control many aspects of plant growth and development. For example extracellular invertase plays a crucial role in source-sink regulation and in supplying carbohydrates to sink tissues. (Tang et al., 1999; Roitsch et al., 2003) It also plays a central role in senescence. The expression of an extracellular invertase under control of the senescence-induced SAG12 promoter results in increased invertase activity in the apoplast and delays in senescence (Balibrea Lara et al., 2004 Plant cell).
Senescence is the natural process of leaf death and resource re-mobilization. It is characterized by a breakdown of cell wall components and membrane disruption leading to cellular de-compartmentalization and the loss of tissue structure. During leaf senescence, nutrients stored in the leaf are remobilized to other parts of the plant. Senescence is also an important response to biotic and a-biotic stresses and enables the recycling of valuable resources during periods of stress. Delaying senescence can have significant impacts on crop yield and quality by extending the photosynthetic period. For example increased levels of cytokinin synthesis, mediated by over-expression of the Agrobacterium IPT gene increased biomass by 40% and increased seed yield by 52%. (Gan and Amasino 1995 Science 270:1986-1988). In Rice, the same IPT gene was expressed under the control of a senescence associated promoter and changes in the cytokinin level led to early flowering and a greater number of emerged panicles.
Sweetness is an important consumer trait that determines the quality, flavor and marketability of fruits and vegetables. The composition and quantity of sugars primarily dictates the degree of sweetness in most fruits and vegetables. The sugar content depends upon the total solids, the PH, the fruit size and the acidity. Fructose and glucose are the major sugars in most fruits. One way to increase the sugar content in fruits is to increase the activity of invertase. In tomato the sugar content is not only important for flavor, but it also is the major contributor to the total soluble solids content, which is a key trait for processing tomatoes. Sugar accumulation is also an important characteristic for grape species and is of major commercial importance for winemakers, grape growers and dried fruit producers. The sugar concentration in wine-making grapes is critical because it's fermentation by yeast produces the alcohol and it contributes to the flavor profile. Increasing the sugar content in grape varieties by increasing invertase activity could significantly enhance the quality of wine grape varieties (Kambiranda, D., H., et al. (2011)). Corn, Rice, peppers, lettuce, sugarcane, tomatoes and melons are some additional examples of crops that would benefit from increased sugars.
One strategy for increasing invertase activity is through down-regulation of negative effectors of invertase. Jin et al, Plant cell 2009, cloned an invertase inhibitor gene, INVINH1 from tomato, which has a 516 nucleotide open reading frame that encodes a 16 kD protein (171 amino acids with a 19 AA signal peptide at the N terminus). In tomato, the INVINH1 gene is expressed in the root, stem, sink and source leaves, the flower and 1, 10 and 20 days after flowering (DAF), with expression highest in the root and 20 DAF. Suppression of the INVINH1 gene in tomato using RNAi resulted in elevated levels of cell wall invertase activity, increased in fruit hexose levels and increases in seed weight. Delays in ABA-induced senescence were also observed in INVINH1 suppressed lines.